1. Field of the Invention
This invention relates to a method for augmenting condensation heat transfer by directly removing from the surface of heat transfer the condensate liquid film formed on the surface of heat transfer by application of a non-uniform electric field to the field of condensation heat transfer.
2. Description of the Prior Art
The phenomenon of condensation heat transfer widely utilized such as in heat exchangers is a highly advantageous form of heat transfer. In the generation of electric power by effective use of waste heat, for example, when the exchange of heat is carried out between two heat exchangers both falling below 100.degree. C. and differing from each other by about 80.degree. C., since the temperature difference is very small compared with those generally utilized for heat exchange treatment, the heat transfer area must be made large to obtain the same amount of heat transfer as can be obtained with greater temperature differences. Consequently, the proportion of the heat exchanger to the whole power plant is increased and the construction cost of the power plant is proportionally increased. A demand for a method which improves the heat transfer coefficient of the heat transfer surface and permits a size reduction of the heat exchanger has thus been generated.
Generally, in condensation heat transfer, there is the frequently encountered problem that when the condensate liquid film formed on the heat transfer surface gradually grows in thickness, subsequent condensation is gradually diminished by the formed liquid film. Since this problem is overcome by keeping the formed condensate liquid film to a very small thickness at all times, it has been suggested to effect partial decrease in the film thickness as by extending the heat transfer surface or by making use of corona wind. The method which involves extended surfaces, however, has a disadvantage that it entails a troublesome and costly processing step and fails to give sufficient improvement in the heat transfer coeffcient compared with the increase in cost. Furthermore, use of an organic heat medium such as Freon 113 is not sufficiently effective by this method because of its lower surface tension.
The method which makes use of corona wind removes the formed condensate liquid by causing the liquid to move along the heat exchange surface. Since, in the course of such movement of the liquid, the liquid film loses its thickness at some portions and gains its thickness at other portions, this method fails to bring about large improvement in the efficiency of heat transfer.
There have also been suggested methods resorting to mechanical means, such as a method which forcibly removes the condensate liquid film formed on the heat transfer surface by means of a wiper and a method which locally decreases the thickness of the liquid film formed on the heat transfer surface by imparting vibrations to the heat transfer surface. Since the devices used for these methods inevitably necessitate incorporation of mechanically operated components, they require maintenance and suffer a disadvantage that the power required for their operation increases in direct proportion to the area of heat transfer surface. These methods, therefore, prove uneconomical.
In the February 1968 issue of the Journal of Heat Transfer, there is a report on pages 98-102 to the effect that, in a system of two cylinders concentrically disposed so as to embrace an annular space between the outer surface of the inner cylinder and the inner surface of the outer cylinder, wherein a heat medium is supplied to the annular space to cool the outer cylinder and a condensate liquid film is consequently formed on the inner surface of the outer cylinder, application of electric potential to the two concentric cylinders serves to considerably improve the heat transfer coefficient over the level otherwise attained. The reported explanation for this improvement postulates that, upon exposure to the electric potential, the condensate liquid film undulates and sheds itself in the form of invisible droplets into the vapor stream.